Method and device for controlling the functioning of a nitrogen oxide trap for an internal combustion engine running on a lean mixture

ABSTRACT

A method in which a first oxygen sensor is disposed on an exhaust pipe upstream from a nitrogen oxide trap, and development of a meaningful signal representative of the signal supplied by the sensor is monitored. A substantial increase of the meaningful signal, which is obtained following a variation resulting from a motor being switched from running on a lean mixture to running on a rich mixture, from a first plate having an essentially constant level is used as an indicator to control an end of a purge process. The method can be applied to diesel engines.

The present invention relates to a method and a device for control ofthe operation of a nitrogen oxides trap for an internal combustionengine running on lean mixture.

More particularly, it is intended for diesel engines, or in general forengines whose operation takes place under conditions of emission ofexhaust gas with relatively low richness.

The use of catalysts of the nitrogen oxides trap type, also commonlyknown as “Nox Trap”, is already known in diesel engines. It correspondsto the desire to avoid discharge of nitrogen oxides in the exhaustgases, since the emission of nitrogen oxides at the engine outlet isincreasing with the current attempts to run the engines on lean mixture,that is under conditions of excess oxygen relative to the fuel andtherefore excess air. In other words, in this type of operation withlean mixture, the richness at the intake is lower than 1, the valuecorresponding to a stoichiometric mixture.

During operation of the engine, a nitrogen oxides trap continuouslycaptures the nitrogen oxides contained in the exhaust gases produced bythe said engine. It is therefore necessary to regenerate the nitrogenoxides trap periodically in order to unburden it, which is achieved byan operation known as purging, during which the nitrogen oxides contentis reduced. It is known to perform this purging periodically,temporarily commanding the richness of the engine to be increased, insuch a way that the richness of the exhaust gases upstream from thenitrogen oxides trap is greater than 1 and that the oxygen concentrationis low, that is, such that the engine generates reducing agents such asHC, CO and H₂, capable of reducing the nitrogen oxides stored on thesubstrate of the trap, by means of the following reactions:NO2+2 H2->½N2+2 H2ONO2+2 CO->½N2+2 CO2(m/4+n)NO2+CnHm->(m/8+n/2)N2+n CO2+m/2 H2O

Termination of the purging can be commanded, for example, at the end ofa predetermined time after the engine was changed over to running onrich mixture. However, that does not make any allowance for the realburden of the nitrogen oxides trap, and there is the risk that eitherpurging will be incomplete or that the phase of running on rich mixturewill be impractically long.

The object of the present invention is to overcome this problem and inparticular to provide a method and means with which it is possible, inoptimal manner, to command a stop to purging of a catalyst of thenitrogen oxides trap type disposed in the exhaust line of an internalcombustion engine that normally runs on lean mixture.

With these objectives in mind, the object of the invention is a methodfor control of the operation of a nitrogen oxides trap for an internalcombustion engine running on lean mixture, wherein purging of the saidnitrogen oxides trap is commanded periodically.

According to the invention, this method is characterized in that a firstoxygen sensor is disposed in the exhaust pipe downstream from thenitrogen oxides trap, and the evolution of a meaningful signalrepresentative of the signal delivered by this sensor is observed, asubstantial increase of this meaningful signal from a first plateau ofsubstantially constant level, reached following a variation subsequentto a changeover of the engine from running on lean mixture to running onrich mixture, being used as the indicator to command the end of purging.

In the method according to the invention, the command to end purging ofthe nitrogen oxides trap is therefore achieved in response to a notableevolution of the meaningful signal representative of the condition ofthe oxygen sensor disposed downstream from the trap, this evolutioncomprising a marked increase of the signal beyond a first plateaureached practically as soon as the beginning of purging and at which thesignal had previously substantially stabilized. The inventors in factobserved that, under substantially constant conditions upstream from thenitrogen oxides trap during purging, the signal delivered by the oxygensensor disposed downstream from the nitrogen oxides trap still underwenta meaningful evolution at the moment when the reduction of all storednitrogen oxides was sufficiently complete.

One difficulty encountered was actually that, in the case of enginesrunning on lean mixture, such as diesel engines, the signal of an oxygensensor disposed downstream from the nitrogen oxides trap shoots upalmost immediately, as soon as purging is initiated, and that thereforethis premature upward shoot obviously could not be used to indicate theend of reduction of the nitrogen oxides stored in the nitrogen oxidestrap.

It is recalled here that lambda sensors normally deliver a signal thatis practically binary as a function of whether or not oxygen is beingdetected. Proportional sensors are certainly more suitable for measuringa range of concentrations or progressive variations, but the signal thatthey are capable of delivering nevertheless varies abruptly if theevolution of the oxygen content is strong and fast.

For engines running on lean mixture, the initiation of purging, whichtakes place by changeover to a substantially richer mode of operation inorder to provide reducing elements capable of reacting with the storednitrogen oxides, leads to a large excess of HC in the gases passingthrough the nitrogen oxides trap and arriving immediately at thedownstream sensor, which itself reacts almost immediately by reachingsaturation.

It was known beforehand, in the case of a gasoline engine running onlean mixture, that the upward shoot of the signal of the sensorindicated the disappearance of oxygen at the outlet of the nitrogenoxides trap, which therefore could be interpreted as the end of purging,that is, the arrival again of a reducing mixture at the sensor, whereaspreviously, during purging, the reducing agents originating from theengine were being consumed by the reduction reactions used for purging.In fact, in this case, the proportion of CO to HC in the exhaust gasesis relatively high, but, since the reducing power of CO is much strongerthan that of HC, a kind of equilibrium is established between theaforesaid reduction reactions, leading to a substantially equivalentparticipation of all reducing agents delivered by the engine in purgingof the nitrogen oxides trap. As a result, the gaseous mixture exitingthe nitrogen oxides trap then remains without effect on the sensor whilethese reactions are occurring, and it is only at the end of thereduction reactions that the reducing agents are then present in excessat the outlet of the nitrogen oxides trap, causing the sensor signal toshoot up, since it is then truly representative of the end of purging.

In contrast to the foregoing, it became apparent to the inventors that,in the case of the engines considered by the present invention,operating with a richness on the order of 1 to 1.1, the upward shoot ofthe signal takes place almost as soon as purging begins following therelatively massive arrival of HC at the sensor disposed downstream fromthe nitrogen oxides trap. An explanation of this phenomenon, which doesnot occur in the case of operation with greater richness, such as 1.2 to1.4, appears to lie in the fact that the HC/CO ratio is higher in theexhaust gases in the case of operation with lower richness. In fact, theevolution of CO content as a function of richness is exponential, whileit is linear for HC. Now, since the reducing power of H₂ and of CO isgreater than that of HC, the purging reactions will therefore take placepreferentially with these H₂ and CO reducing agents but, because of therelative excess of HC, the equilibrium of reactions mentioned in theforegoing no longer is established, thus leading to the presence ofunreacted HC at the outlet of the nitrogen oxides trap practically assoon as purging begins. The variation of the signal delivered by thesensor therefore no longer is indicative of the total reduction of thenitrogen oxides contained in the nitrogen oxides trap, but to thecontrary is evident as soon as purging begins.

The inventors therefore searched for another means of detecting this endof purging. They discovered that, surprisingly, since in particular thelambda sensors are considered to deliver a quasi-binary signal, thissignal delivered by an oxygen sensor underwent a new variation startingfrom a substantially constant plateau at which the arrival of the HC hadcaused the signal to shoot up as soon as purging began.

An analysis of this phenomenon led to the observation that, after thesignal arrived in this way at the level of the first plateau and wasmaintained there during purging as long as the aforesaid reductionreactions did not treat the majority of the nitrogen oxides stored onthe catalyst substrate of the nitrogen oxides trap, this signal shiftedto a second plateau of higher level, substantially at the moment of theend of the said reduction reactions, and then remained at this level aslong as the engine continued to run on rich mixture. The inventors thenimagined taking advantage of this evolution of the shift of the signalfrom the first to the second plateau as an indicator of the end ofpurging and thus for commanding automatically the return to running theengine on normal mixture, that is, on lean mixture.

One explanation of this phenomenon is that, during the purging phase,the quantity of hydrogen downstream from the nitrogen oxides trap isclose to zero, because the hydrogen upstream is consumed almostcompletely by the catalytic reactions, the reducing power of H₂ beingpredominant, as already indicated hereinabove. When the reductionreactions are terminated by virtue of the small residual quantity ofnitrogen oxides stored in the nitrogen oxides trap, the HC, CO and H₂reducing agents will be found in increasing concentration downstreamfrom the nitrogen oxides trap and simultaneously will favor theformation of H₂ within the nitrogen oxides trap via various mechanisms,including in particular:H₂O+CO->H₂+CO₂

This formation of H₂ was measured by gas-phase chromatography andcontinuous mass spectrometry.

This modification of the concentration of reducing agents downstreamfrom the nitrogen oxides trap influences the signal of oxygen sensors ofall-or-nothing type, such as the lambda sensors or the sensors ofproportional type, such as the sensors known under the name of UEGO®sensor, which then present a signal in the form of the said secondplateau situated at a level higher than the first plateau. The oxygensensors of the lambda type and the oxygen sensors of the proportionaltype are particularly sensitive to hydrogen present in the exhaustgases, and they exhibit a large variation of their signal when the H2concentration in the exhaust gases changes and therefore during theshift from the first to the second plateau.

This particular feature therefore made it possible to take advantage ofthe variation of the signal from its first plateau to detect the end ofpurging and consequently to command the engine to return to running onpoor mixture, if necessary with a certain time delay after detection ofthe variation proper.

According to a particular arrangement of the invention, there isadditionally used a second oxygen sensor disposed upstream from thenitrogen oxides trap, in order to deliver a reference signal relative towhich the evolution of the signal delivered by the first sensor iscompared in order to deliver the said meaningful signal. The use of thisreference signal, which can be influenced by variations of the operatingconditions of the engine independent of those programmed for the purgingoperation proper, therefore makes it possible, by comparison, toreliably detect the variation that is indicative of the end of thereduction reactions in the nitrogen oxides trap.

In addition, with oxygen sensors of the same type upstream anddownstream from the nitrogen oxides trap, for example two lambda sensorsor two proportional sensors, the sensitive element of which istemperature-controlled or else the signal of which is corrected as afunction of the temperature of the sensitive element, the higher H₂concentration in the gases downstream than in the gases upstream fromthe nitrogen oxides trap is manifested by a higher signal for the sensorsituated downstream than for the sensor situated upstream when thereduction of the stored nitrogen oxides is sufficiently advanced.

As a function of the H₂ concentration upstream, and in the case in whichtwo lambda sensors are used, the signal of the upstream lambda sensorcan be at the same level as the signal of the downstream lambda sensorat the second plateau.

Traditionally, the oxygen sensor or sensors will be chosen from amongthe sensors of the following type: sensor of lambda type, proportionaloxygen sensor, nitrogen oxides detector, in which the capability ofmeasuring the oxygen concentration is used.

In the case of use of a sensor upstream and of a sensor downstream fromthe nitrogen oxides trap, it will also be possible for the two sensorsto be of different types, and particular, it will then be possible touse, for the purposes of the invention, sensors that may already beinstalled in the exhaust line to perform other functions. In particular,it will be possible to use the different combinations listed in thefollowing table. Upstream sensor Downstream sensor Proportional sensorLambda sensor Proportional sensor O2 function of the Nox detector Lambdasensor Proportional sensor Lambda sensor O2 function of the Nox detectorO2 function of the Nox detector Lambda sensor O2 function of the Noxdetector Proportional sensor

where “O2 function of the Nox detector” denotes the use of theoxygen-concentration measuring function of a nitrogen oxides detector,as indicated in the foregoing.

When two oxygen sensors of different nature are used, it is necessaryeither to introduce a transfer function between the sensor signals(taking into account any differences in response times) or to use cellsequivalent to that of the lambda sensor.

According to a first embodiment, the increase of the meaningful signalis detected by applying filtering of the first derivative of themeaningful signal and by comparing the filtered first derivative with apredetermined threshold.

According to a second embodiment, the increase of the meaningful signalis detected by applying filtering of the second derivative of themeaningful signal and observing the passage of the filtered secondderivative through zero in decreasing threshold.

According to a third embodiment, the increase of the meaningful signalis detected by taking the difference between the instantaneous value ofthe meaningful signal and a sliding mean of the said signal, and bycomparing this difference with a threshold.

According to a fourth embodiment, the increase of the meaningful signal,for a lambda sensor, is detected by comparing the voltage valuedelivered by the sensor with a predetermined threshold.

A practical example of the invention will now be described for a dieselengine.

Reference will be made to the attached drawings, wherein:

FIG. 1 is a schematic and partial representation of the gas circuit ofthe said engine,

FIG. 2 is a graph that shows the curves representative of the signal ofdifferent sensors disposed upstream and downstream from the nitrogenoxides trap, and that illustrates the variation of the signal used asindicator of the end of purging.

On FIG. 1 there are schematically represented the elements of the enginethrough which the gases pass, and there are seen therein, successivelyin the direction of flow of the gases:

-   -   inlet 11 for air arriving from the air filter,    -   suction part 12 of a turbocompressor,    -   intake manifold 13,    -   the upper part of cylinder 14, equipped with an injector 15,    -   exhaust tubes 16 and the exhaust part 17 of the turbocompressor,    -   nitrogen oxides trap 18,    -   the exhaust line, equipped with a particle filter 19.

A first oxygen sensor 21 is mounted downstream from the nitrogen oxidestrap, and a second oxygen sensor 22 is mounted upstream from thenitrogen oxides trap, the two sensors being connected to a calculatingunit 23, which itself is connected to an engine-control unit 24.

Sensors 21 and 22 are, for example, proportional or lambda sensors.

The graph of FIG. 2 shows the results of test measurements performedwith different types of sensors as a function of the time frominitiation of purging. On the ordinate, the left scale represents thevalue of the signal delivered by a sensor, and the right scalerepresents the quantity of CO downstream from the nitrogen oxides trap(for trace 6).

Line 3 represents the signal delivered by a sensor of lambda typedisposed downstream from the nitrogen oxides trap. On the line there isclearly seen the upward shoot 31 of the sensor as soon as purgingbegins, and the first plateau 32 which remains substantially stableduring purging, then the marked variation 34 of the signal,representative of the arrival of H₂ at the sensor and is thereforeindicative of the end of purging, before the signal restabilizes at asecond plateau 33.

Line 4 represents the signal delivered by a sensor of proportional typedisposed downstream from the nitrogen oxides trap. On the line there isclearly seen the upward shoot 41 of the sensor as soon as purgingbegins, and the first plateau 42 which remains substantially stableduring purging, then the marked variation 44 of the signal,representative of the arrival of H₂ at the sensor and is thereforeindicative of the end of purging, before the signal restabilizes at asecond plateau 43.

In comparison, line 5 represents the signal delivered by a sensor ofproportional type disposed upstream from the nitrogen oxides trap. It isclearly seen that this signal also undergoes an abrupt rise 51 at themoment of initiation of purging, due to the influx of reducing agents inthe exhaust gases. In contrast, the signal then remains constant on aplateau 52, as long as the engine is maintained under constantconditions of running on rich mixture. In the event of any perturbationwhatsoever of these conditions during purging, that perturbation willcorrespondingly affect both the upstream sensor and the sensor situateddownstream from the nitrogen oxides trap, and it will be possible to usethis signal delivered by the upstream sensor as a reference forevaluating, by comparison, the variations of the signal of thedownstream sensor, truly indicative of the end of purging.

It will also be noted that the level of plateau 42 is situated belowplateau 52, the difference corresponding to the fraction of the reducingagents used to reduce the nitrogen oxides stored in the filter. Incontrast, when purging is terminated, the signal of the downstreamsensor shifts above that of the upstream sensor, clearly illustratingthe arrival at the sensor of the hydrogen formed in the nitrogen oxidestrap by the aforesaid reaction mechanism.

On this graph there was also represented a line 6 representing theevolution of the CO content downstream from the nitrogen oxides trap,measured in experimental manner, and the evolution of which, marked bythe strong increase 61 at the moment when the sensor signals shift fromthe first to the second plateau, clearly corroborates the end ofconsumption of the said CO for reducing the nitrogen oxides stored inthe nitrogen oxides trap, and therefore the advanced disappearancethereof.

As an example of how the signal obtained from the sensors is processed,there was also represented in the graph the line 7, which represents thefiltered first derivative of the signal, and illustrates the possibilityof detecting the end of purging by comparing this signal with apredetermined threshold, crossed by the said derivative during itsabrupt increase 71, corresponding to the increase of the slope of thesignal obtained from the sensor.

Similarly, there was also represented the line 8, which represents thesecond derivative, whose passage through zero in decreasing threshold 81can also be used as information indicative of the end of purging.

The invention is not limited to the examples described hereinabove. Itis intended more particularly for diesel engines, but it could also beapplied in general to all engines that normally run on lean mixture.

1-10. (canceled)
 11. A method for control of operation of a nitrogen oxides trap for an internal combustion engine running on a lean mixture, wherein purging of the nitrogen oxides trap is commanded periodically, and a first oxygen sensor is disposed in an exhaust pipe downstream from the nitrogen oxides trap, the method comprising: observing evolution of a meaningful signal representative of a signal delivered by the first oxygen sensor; using an increase of the meaningful signal from a first plateau of substantially constant level, reached following a variation subsequent to a changeover of the engine from running on a lean mixture to running on a rich mixture, as an indicator to command an end of purging.
 12. A method according to claim 11, wherein a second oxygen sensor disposed upstream from the nitrogen oxides trap is additionally used to deliver a reference signal, relative to which the evolution of the signal delivered by the first oxygen sensor is compared to deliver the meaningful signal.
 13. A method according to claim 11, wherein the increase of the meaningful signal is detected by applying filtering of a first derivative of the meaningful signal and by comparing the filtered first derivative with a predetermined threshold.
 14. A method according to claim 12, wherein the increase of the meaningful signal is detected by applying filtering of a first derivative of the meaningful signal and by comparing the filtered first derivative with a predetermined threshold.
 15. A method according to claim 11, wherein the increase of the meaningful signal is detected by applying filtering of a second derivative of the meaningful signal and observing passage of the filtered second derivative through zero in decreasing threshold.
 16. A method according to claim 12, wherein the increase of the meaningful signal is detected by applying filtering of a second derivative of the meaningful signal and observing passage of the filtered second derivative through zero in decreasing threshold.
 17. A method according to claim 11, wherein the increase of the meaningful signal is detected by taking a difference between an instantaneous value of the meaningful signal and a sliding mean of the meaningful signal, and by comparing the difference with a threshold.
 18. A method according to claim 12, wherein the increase of the meaningful signal is detected by taking a difference between an instantaneous value of the meaningful signal and a sliding mean of the meaningful signal, and by comparing the difference with a threshold.
 19. A method according to claim 11, wherein the increase of the meaningful signal, for a lambda sensor, is detected by comparing a voltage value delivered by the first oxygen sensor with a predetermined threshold.
 20. A method according to claim 12, wherein the increase of the meaningful signal, for a lambda sensor, is detected by comparing a voltage value delivered by the first oxygen sensor with a predetermined threshold.
 21. A method according to claim 11, wherein the first oxygen sensor is chosen from among sensors of a sensor of lambda type, proportional oxygen sensor, nitrogen oxides detector, in which the oxygen-concentration measuring function is used.
 22. A method according to claim 12, wherein the first oxygen sensor is chosen from among sensors of a sensor of lambda type, proportional oxygen sensor, nitrogen oxides detector, in which the oxygen-concentration measuring function is used.
 23. A method according to claim 22, wherein the first and second oxygen sensors are of different types.
 24. A device for control of an operation of a nitrogen oxides trap for an internal combustion engine running on a lean mixture, for use of the method according to claim 11, the engine being equipped with an exhaust line containing a nitrogen oxides trap, the device comprising: a first oxygen sensor disposed on an exhaust line downstream from the nitrogen oxides trap; and calculating means for determining an increase of a meaningful signal representative of the signal delivered by the first oxygen sensor from a first plateau of substantially constant level, reached following initiation of a purging operation, and using the increase as an indicator to command an end of purging.
 25. A device according to claim 24, further comprising a second oxygen sensor disposed upstream from the nitrogen oxides trap and connected to the calculating means to deliver a reference signal thereto. 